Smartphones and other mobile devices have rapidly become ubiquitous throughout the world. Mobile phones and tablet computers are commonly seen in use at restaurants, in waiting rooms, or on street corners. Mobile devices are used for gaming, photography, listening to music, social networking, or simply talking with another person via a built in microphone and speaker.
Mobile devices enrich lives by keeping family and friends in communication, allowing any moment to be captured as a photo or video, and providing a means of contacting someone in an emergency situation. At the same time, mobile devices pose certain dangers to users. Accidents occur when a driver is distracted by an incoming text message or ongoing phone call. Pedestrians are injured or killed due to paying closer attention to a mobile device than to nearby traffic. In addition, the potential exists that radiation emanating from a mobile device will be absorbed by a human body and cause damage to the health of a user.
FIG. 1a illustrates a mobile device 10. Mobile device 10 is a touchscreen slate cellular (cell) phone. In other embodiments, mobile device 10 is a tablet computer, pager, GPS receiver, smartwatch or other wearable computer, laptop computer, handheld game console, or any other device capable of radio communication.
Mobile device 10 includes touchscreen 12 on a front side of the mobile device. Touchscreen 12 is used to display a graphical user interface (GUI). The GUI on touchscreen 12 presents feedback, notifications, and other information to a user as determined by an operating system of mobile device 10. Touchscreen 12 is sensitive to physical touch from body parts of a user of mobile device 10. Touchscreen 12 utilizes resistance, capacitance, acoustic waves, an infrared grid, optical imaging, or other methods to determine the presence and location of a user's touch.
In one common usage scenario of mobile device 10, touchscreen 12 displays a button as a part of the GUI, and a user touches the location of the button on the touchscreen to perform an action associated with the button. In one embodiment, touchscreen 12 displays a 3×4 telephone keypad. A user dials a telephone number on the displayed keypad by touching touchscreen 12 at the locations where the desired numbers to dial are displayed. Touchscreen 12 displays a keyboard including buttons for alphabetical, numeric, and punctuation characters, along with, or as an alternative to, the telephone keypad, with a user touching the touchscreen in the location of letters, numbers, or symbols to be entered in a text input field displayed on the touchscreen. Touchscreen 12 is also used to watch downloaded or streamed videos, or play games, with a user's touch controlling playback of the video or play of the game. In some embodiments, touchscreen 12 is sensitive to a user's touch when the display component of the touchscreen is disabled. While listening to music, a user pauses the music, or advances to the next track of music, by drawing a symbol on touchscreen 12 even though nothing is displayed on the touchscreen.
Buttons 14 provide an alternative user input mechanism to touchscreen 12. Buttons 14 perform functionality depending on the programming of the operating system running on mobile device 10. In one embodiment, buttons 14 return the GUI on touchscreen 12 to a home screen, go back to a previous GUI screen, or open up a menu on the GUI. In other embodiments, the functionality of buttons 14 changes based on a context displayed on touchscreen 12.
Speaker 16 provides audible feedback to a user of mobile device 10. When mobile device 10 receives an incoming message, speaker 16 produces an audible notification sound to alert a user to the received message. An incoming telephone call causes a ringing sound from speaker 16 to alert the user. In other embodiments, a musical ringtone, selectable via the GUI on touchscreen 12, is played via speaker 16 when an incoming telephone call is received. When mobile device 10 is used to participate in a telephone call, a user of the mobile device speaks into microphone 17 while the other conversation participants' voices are reproduced by speaker 16. When a user watches a movie or plays a game, the sound associated with the movie or game is produced by speaker 16 for the user to hear.
Front facing camera 18 provides visual feedback to the operating system of mobile device 10. Camera 18 creates a digital image of the area facing touchscreen 12. Camera 18 is used in video chat applications running on mobile device 10 to capture video of a user's face during a conversation. Mobile device 10 transmits the video of a user to another person in a remote location and receives a streaming video of the other person which is displayed on touchscreen 12. Camera 18 is also used to take selfies or other pictures. When camera 18 is used to take pictures, touchscreen 12 displays the image being captured by the camera so that the touchscreen is an electronic viewfinder. Captured photographs are stored on memory within mobile device 10 for subsequent viewing on touchscreen 12, sharing on social networks, or backing up to a personal computer.
Housing 20 provides structural support and protection for the internal components of mobile device 10. Housing 20 is made of rigid plastic or metallic materials to withstand environmental hazards which cause harm to the circuit boards and other components within mobile device 10 if exposed directly. In one embodiment, a panel of housing 20 opposite touchscreen 12 is removable to expose interchangeable parts of mobile device 10 such as a subscriber identification module (SIM) card, flash memory card, or battery. Housing 20 includes a transparent glass or plastic portion over touchscreen 12, which protects the touchscreen from environmental factors while allowing a user's touch to be sensed through the housing.
FIG. 1b illustrates a user 30 operating mobile device 10 as a telephone. A portion of housing 20 is removed to illustrate antenna 32 within mobile device 10. User 30 holds mobile device 10 with speaker 16 over an ear of the user. Microphone 17 is oriented toward a mouth of user 30. When user 30 speaks, microphone 17 detects and digitizes the user's voice for transmission to a person the user is speaking with. The person that user 30 is speaking with transmits a digitized voice signal to mobile device 10 which is reproduced on speaker 16 and heard by the user. User 30 thereby converses with another person using mobile device 10.
Mobile device 10 sends a voice signal of user 30, and receives a voice signal of a person being conversed with, using a cellular network or other network capable of voice traffic. In various embodiments, mobile device 10 transmits voice signals and other data over Wi-Fi, Bluetooth, GSM, CDMA, LTE, HSPA+, WiMAX, or other wireless network types. Mobile device 10 transmits a voice signal using radio frequency (RF) electromagnetic waves emanating from RF antenna 32. An RF amplifier in mobile device 10 supplies an electric current, which contains the voice information and oscillates at radio frequencies, to antenna 32. Antenna 32 radiates energy of the current as electromagnetic waves through the surrounding atmosphere. The electromagnetic waves reach a cellular tower which forwards the voice signal on to ultimately be received by the person that user 30 is conversing with.
FIG. 1c is a block diagram of an RF section 33 of mobile device 10. RF section 33 represents a portion of the circuitry located on a circuit board within mobile device 10. RF section 33 includes microcontroller or central processing unit (CPU) 34, RF transceiver 36, RF amplifier 38, and antenna 32. For mobile device 10 to receive an audio signal or other digital data, radio waves are first received by antenna 32. Oscillating electric and magnetic fields of an incoming radio wave exert force on electrons in antenna 32, causing the electrons to oscillate and creating a current in the antenna. RF transceiver 36 demodulates the incoming signal to eliminate RF signals and sends the underlying data to CPU 34.
When mobile device 10 is transmitting data, CPU 34 first provides data to be transmitted. In one embodiment, CPU 34 receives audio data from microphone 17 and performs digital signal processing functions on the audio data. CPU 34 performs any digital signal processing or baseband processing required for the audio data, or a separate digital signal processor (DSP) or baseband integrated circuit (IC) is used. In other embodiments, non-voice data is sent, e.g., an outgoing text message or a uniform resource locator (URL) of a website which user 30 wishes to view on touchscreen 12. Once CPU 34 has received or generated the data to be transmitted, the data is sent from the CPU to RF transceiver 36. RF transceiver 36 generates an RF signal containing the data to be transmitted by modulating the data using the frequency for a network that mobile device 10 is communicating with.
The RF signal is sent from RF transceiver 36 to RF amplifier 38. RF amplifier 38 amplifies the signal from RF transceiver 36 to generate a higher power RF signal for transmission by antenna 32. RF amplifier 38 sends the amplified RF signal to antenna 32. The amplified RF signal causes an oscillating current of electrons within antenna 32. The oscillating electric current creates an oscillating magnetic field around antenna 32 and an oscillating electric field along the antenna. The time-varying electric and magnetic fields radiate away from antenna 32 into the surrounding environment as an RF electromagnetic wave.
The output power of RF amplifier 38 is controlled by CPU 34. CPU 34 controls the strength of an RF signal emanating from antenna 32 by configuring a gain setting of RF amplifier 38. A device receiving radio waves from mobile device 10 can be from a few feet away for in-home Wi-Fi, to a few miles away for rural cellular service, or potentially even further away from the mobile device. A higher gain setting of RF amplifier 38 causes a higher power electromagnetic radio wave to emanate from mobile device 10. A higher power electromagnetic radio wave is received at a location further away from mobile device 10.
Antenna 32 is omnidirectional, i.e., the antenna radiates energy approximately equally in every direction from mobile device 10. An omnidirectional antenna 32 gives mobile device 10 good connectivity with a cellular tower without regard to the angle the mobile device is held at. However, due to the omnidirectional nature of antenna 32, a significant amount of RF electromagnetic radiation from the antenna is radiated into user 30 when the user holds the mobile device near a body part, as illustrated in FIG. 1b. Some health concerns exist in relation to RF radiation from mobile devices, such as mobile device 10, being absorbed by the human body. Some studies suggest that RF energy absorbed by the body of may be linked to cancer and other illnesses.
Specific absorption rate (SAR) is a measure of the rate at which energy is absorbed by the human body when exposed to an RF electromagnetic field. SAR measures exposure to electromagnetic fields between 100 kHz and 10 GHz. A SAR rating is commonly used in association with cell phones and magnetic resonance imaging (MRI) scanners.
When measuring SAR due to mobile device 10, the mobile device is placed at the head in a talk position, as illustrated in FIG. 1b. The SAR value is then measured at the location that has the highest absorption rate in the entire head, which is generally the closest portion of the head to antenna 32. In the United States, the Federal Communications Commission (FCC) requires that mobile devices have a SAR level at or below 1.6 watts per kilogram (W/kg) taken over the volume containing a mass of 1 gram of tissue that is absorbing the most RF energy. In Europe, the European Committee for Electrotechnical Standardization (CENELEC) specifies a SAR limit of 2 W/kg averaged over the 10 grams of tissue absorbing the most RF energy.
Regulations limiting the SAR from mobile device 10 in effect limit the RF power of the mobile device when in use near the body of user 30. Limiting RF output limits signal strength and can harm connectivity of mobile device 10 to cell phone towers. FIGS. 2a-2c show graphs of SAR versus the distance of mobile device 10 from user 30. In FIGS. 2a and 2b, RF amplifier 38 has a constant power output. In FIG. 2a, CPU 34 has configured RF amplifier 38 for high RF power and good connectivity of mobile device 10 to cell phone towers. Line 40 illustrates that with a constant RF power output, SAR is reduced as mobile device 10 is moved further away from user 30, i.e., further right on the graph in FIG. 2a. As mobile device 10 is moved closer to user 30, SAR increases.
Radiation emanating from mobile device 10 attenuates, or reduces in magnitude, as the radiation travels further away from antenna 32. When mobile device 10 is directly next to the head of user 30, much of the radiation emanating from antenna 32 is concentrated on a small area of the head, resulting in a high SAR. When mobile device 10 is further away from user 30, radiation spreads out and hits a larger area of the user's body at a lower energy level. Much of the radiation which hits user 30 when mobile device 10 is held up to the head will miss the user when the mobile device is held at a distance.
Line 40 shows that when configured for high RF power and good connectivity, mobile device 10 will exceed SAR regulatory limit 42 when the mobile device is held within a distance d of a body part of user 30. In one embodiment, the distance d at which mobile device 10 exceeds SAR regulatory limit 42 when configured for high power output is 10 millimeters (mm). Mobile device 10 as configured in FIG. 2a includes good connectivity but is out of compliance with SAR regulations.
One solution to ensure that the SAR of mobile device 10 remains under regulatory limit 42 is to reduce the RF output power of the mobile device, illustrated by FIG. 2b. Line 44 shows that as mobile device 10 is moved further away from user 30, SAR is reduced, as with the configuration of FIG. 2a. However, in FIG. 2b, mobile device 10 is configured for a lower RF output, and does not exceed SAR regulatory limit 42 when the mobile device is held against user 30. The lower RF output makes mobile device 10 in compliance with SAR regulations, but reduces connectivity of the mobile device.
FIG. 2c illustrates another solution to maintaining the SAR of mobile device 10 under regulatory limit 42. When mobile device 10 is held at a distance greater than d from user 30, RF power output of the mobile device, illustrated by line 46, is at a level similar to the higher power setting illustrated in FIG. 2a. When mobile device 10 is moved within a distance d of user 30, i.e., the distance at which SAR would exceed regulatory limit 42 in the configuration of FIG. 2a, the RF output of the mobile device is reduced to remain under the regulatory limit. The reduced RF output within distance d is illustrated by line 48, which is similar to line 44 of FIG. 2b. As configured in FIG. 2c, mobile device 10 includes good connectivity when held a distance greater than d from user 30, and a reduced RF output to remain under SAR regulatory limit 42 when held within a distance d of the user.
To implement the configuration illustrated in FIG. 2c, mobile device 10 includes a proximity sensor used to detect distance from user 30. When the proximity sensor detects user 30 is within a distance d of the proximity sensor, CPU 34 reduces the RF power output of RF amplifier 38 to prevent SAR from rising above regulatory limit 42. When the proximity sensor detects no human body within a distance d of mobile device 10, CPU 34 increases RF power output to improve connectivity.
One goal of mobile device manufactures is to improve the accuracy of proximity sensors. Inaccurate proximity readings result in a high power mode of mobile device 10 being enabled within a distance d of user 30, in violation of SAR regulations. Inaccurate proximity readings also result in a low power mode of mobile device 10 being enabled outside a distance d of user 30, resulting in an unnecessary degradation of connectivity. An accurate proximity sensor provides for an immediate decrease of RF power output when a mobile device is moved within a distance d of a human body, and an immediate increase of RF power output when the mobile device is moved outside of a distance d of the human body.
Achieving accurate proximity readings is difficult when a proximity sensor is near antenna 32 while the antenna is being used for RF transmissions. RF signals used in transmissions over antenna 32 cause interference in capacitive touch sensing because of RF coupling between the antenna and sensing elements of the proximity sensor. Due to the protocols used in RF communication via antenna 32, time multiplexing RF communication and capacitive sensing is difficult. RF communication occurs at the same time as capacitive touch sensing. RF energy from antenna 32 reaches the IC performing the capacitive touch sensing and interferes with an accurate reading of the proximity of a user.